Aviation safety investigations & reports

Boeing Co 737-476, VH-TJY

Summary

A Boeing 737 (B737) registered VH-TAW (TAW) was inbound to Ayers
Rock from Sydney at flight level (FL) 320. A B737 registered VH-TJY
(TJY) departed Ayers Rock for Sydney with a planned level of FL350.
The Alice Springs sector controller calculated the estimated time
of passing for the aircraft as 1328 Central Standard Time (CST).
The controller entered FL310 into The Australian Advanced Air
Traffic Control System (TAAATS) for TJY but unintentionally
instructed the crew to "Climb to amended FL330". At 1321 CST, after
the crew of TAW had reported on the sector frequency, the
controller requested them to "Report sighting and passing TJY on
climb to FL310". The crew of TAW responded that they would advise.
The crew of TJY heard the controller's transmission and queried
their assigned level of FL330. The controller advised that crew
that they had been assigned FL310. The crew of TJY, having passed
FL320, elected to continue the climb and at 1322 CST they reported
maintaining FL330. The application of standard separation required
the aircraft to be established 1,000 ft vertically apart ten
minutes prior to the estimated time of passing. There was an
infringement of separation standards.

Reduced vertical separation minimum (RVSM) procedures had been
introduced across the Australian airspace on 1 November 2001. Those
procedures reduced the previous 2,000 ft vertical separation
standard for aircraft operating above FL290 to 1,000 ft for
approved aircraft operating between FL290 and FL410. Both aircraft
were RVSM approved and the controller had undergone RVSM training
prior to the change in procedure.

The controller had seven years experience in air traffic control
and was rated on the Alice Springs sector in June 2000. During the
12 months prior to the incident the controller had spent the
majority of his time working on that sector. The controller was
included on the team leader roster in September 2001.

The Ayers Rock sector position was located in the Melbourne Air
Traffic Control centre. Team leaders worked a daily shift from 0830
to 1630 Eastern Summer Time (ESuT). The incident shift was the
seventh day of the controller's shift period. After arriving at
work the controller, as the team leader, was advised that a
rostered controller was unavailable. He unsuccessfully sought
approval to call in a replacement controller. The controller then
self-briefed and from 0900 to 1030 ESuT operated one of the Group's
control positions. The controller had a break during which he
endeavoured to resolve the controller shortfall by roster
adjustments before returning to another operating position at 1100
ESuT. The controller had scheduled a 1330 ESuT meeting for a
project he was working on and organised his periods at the console
to ensure that he was able to attend that meeting. He took a second
break at 1230 ESuT before taking over the Alice Springs sector
position at 1300 ESuT. He reported that he had lunch during one of
the breaks when he left the operations room for about 10
minutes.

The controller reported that there was a medium level of air
traffic. The Alice Springs sector was combined with the low-level
Todd sector. That required the controller to operate on three radio
frequencies. There were also several aircraft on frequency with
similar callsigns, including aircraft registered: VH-TJY, VH-TJJ,
VH-TAW and VH-TJD. Each of those aircraft required separation
action or clearance adjustment. It was during that time that the
controller made a communication error in that a crew was addressed
by an incorrect callsign. That error was undetected but did not
affect safety.

The crew of TJY had planned to operate at FL350 and the
controller was aware that there was insufficient time to establish
the required passing standard. He intended to maintain TJY at FL310
until it had passed TAW. The crew of TJY reported departure from
Ayers Rock to the controller at 1311 and shortly after, were
cleared to enter controlled airspace "On track to Oodnadatta, and
planned route, on climb to amended FL330". The crew read back that
clearance. The controller used the cleared flight level field in
the aircraft's label on the air situation display to change the
level to FL310. Analysis of the recorded system and audio data
confirmed that the controller had entered, and accepted, FL310 in
TAAATS and had transmitted FL330 to the crew.

Analysis

It was likely that the controller was distracted by thoughts of
the pending meeting. He may also have been fatigued to some degree
as he had been occupied with either operational or administrative
tasks since commencing work and had not had an adequate rest
period.

It is possible that the controller did not appreciate the error,
because the read back by the pilot phonologically matched the
information stored in the controller's short-term memory. The
controller had issued a clearance of FL330 to the crew and this was
the same flight level that was read back.

The aviation system relied on the instruction and readback cycle
used by pilots and controllers to prevent misunderstanding. The
nature of interacting with TAAATS can make the controller response
to the clearance read back an automatic process that provides no
clue to input errors. While the read back process used two parties,
a pilot and a controller, to challenge and check each other, the
data input to TAAATS was reliant on controller self-checking. The
occurrence highlighted the limitations inherent in using
individuals to check their own work. Controllers need to be aware
of the potential for error when checking TAAATS data inputs against
clearance read backs.

The situational awareness of the crew of TJY, and their query
regarding the assigned flight level, ensured that the situation was
clarified and safety was maintained.

Safety Action

The investigation found a safety deficiency relating to the
limitations of self-checking of data inputs by controllers. Any
outcome from the investigation of the safety deficiency will be
published on the Australian Transport Safety Bureau website
www.atsb.gov.au.